A human spine has three main regions—the cervical, thoracic, and lumbar regions. In a normal spine, the cervical and lumbar regions have a lordotic (backward) curvature, while the thoracic region has a kyphotic (forward) curvature. Such a disposition of the curvatures gives a normal spine an S-shape. Sagittal imbalance is a condition in which the normal alignment of the spine is disrupted in the sagittal plane causing a deformation of the spinal curvature. One example of such a deformity is “flat-back” syndrome, wherein the lumbar region of the spine is generally linear rather than curved. A more extreme example has the lumbar region of the spine exhibiting a kyphotic curvature such that the spine has an overall C-shape, rather than an S-shape. Sagittal imbalance is disadvantageous from a biomechanical standpoint and generally results in discomfort, pain, and an awkward appearance in that the patient tends to be bent forward excessively.
Various treatments for sagittal imbalance are known in the art. These treatments generally involve removing at least some bone from a vertebra (osteotomy) and sometimes removal of the entire vertebra (vertebrectomy) in order to reduce the posterior height of the spine in the affected region and recreate the lordotic curve. Such procedures are traditionally performed via an open, posterior approach involving a large incision (often to expose multiple spinal levels at the same time) and require stripping of the muscle tissue away from the bone. These procedures can have the disadvantages of a large amount of blood loss, high risk, long operating times, and a long and painful recovery for the patient.
In some other treatments, achieving sagittal balance is accomplished by via an open, anterior approach to position an intervertebral implant between two affected vertebrae in order to increase the anterior height of the spine in the affected region and thereby recreate the lordotic curve. Effectuating an anterior spinal fusion typically involves retracting the great vessels (aorta and vena cava) and tissue adjacent to the anterior longitudinal ligament (ALL), then severing the ALL 16 to increase flexibility and permit insertion of the implant between the adjacent vertebrae. The anterior approach is advantageous in that the ALL 16 is generally exposed, allowing the physician to simply dissect across the exposed portion of the ALL 16 to access the spine. The anterior approach to the spine can also have the disadvantages of a large amount of blood loss, build-up of scar tissue near vital organs, and sexual dysfunction in males. Furthermore, depending upon the patient, multiple procedures, involving both anterior and posterior approaches to the spine, may be required.
In contrast, a lateral approach could be used to access a target spinal site, remove the intervertebral disc between two affected vertebrae, and insert an intervertebral implant. A lateral approach to the spine provides a number of advantages over the posterior and anterior approaches to the spine. Because a lateral approach may be performed without creating a large incision or stripping muscle from bone, this approach does not present the problems associated with a posterior approach, namely there is no large incision, muscle stripping, high blood loss, long operating time, or long and painful recovery for the patient. Furthermore, because a lateral approach to the spine does not involve exposing the anterior aspect of the ALL 16, retracting the great vessels and nearby tissues is unnecessary such that the risks of blood loss, scar tissue, and sexual dysfunction are much less likely to be encountered.
However, in patients with sagittal imbalance, release of the ALL 16 may be necessary to achieve the flexibility between the two affected vertebrae to facilitate insertion of an implant and achieve the amount of correction desired. A need exists for implants, tools, and methods for safe and reproducible means of releasing the ALL 16 via lateral approach as well as restoring the lordotic curvature of the lumbar spine. The present invention is directed at overcoming, or at least improving upon, the disadvantages of the prior art.